The present investigation analyzed rat striatal tyrosine hydroxylase in vitro at pH 7.2 using the natural cofactor 6(R) L-erythrotetrahydrobiopterin. Both control enzyme activity and activated enzyme were examined. The results showed that there are two different forms of the enzyme. One form has an apparent Km of 240 muM and upon phosphorylation is converted to 8 muM. The other form has a Km of 1.4 mM and appears unaffected by phosphorylation. In addition, enzyme activity was not linear for time during the first 10-15 min of incubation. This non-linearity was not apparent at low cofactor concentrations using enzyme activated by phosphorylation.